The present invention relates generally to an estimating device for vehicles and, more particularly, to a vehicle estimating device which estimates the road surface coefficient of friction and the slope or grade of a hill a vehicle is climbing.
It is well known in the art to apply anti-skid control systems to vehicles to achieve a satisfactory rotational behavior of the wheels of a motor vehicle during a braking operation. Anti-skid systems are typically designed to increase and reduce brake fluid pressure in a desirable manner to prevent the wheels from becoming locked. Known anti-skid braking control systems use various mathematical manipulations to determine the speed of a vehicle for comparison with the rotational speed of a given vehicle wheel to calculate wheel "slip" for that wheel, wheel slip being a parameter needed in order to make decisions concerning wheel lock control.
Additionally, it is known to provide traction control strategies which attempt to maximize vehicle acceleration and prevent overspin of the wheels. Since the surface condition of a road and the slope or grade of the road surface directly affect braking and traction, the efficiency of control devices such as braking control devices and traction control devices, could be improved if there were a method or apparatus capable of determining road surface coefficient of friction and hill slope.
Of particular interest in both braking and traction control devices is the road surface coefficient of friction, since wheel locking tends to occur when the motor vehicle is braking on a road surface With a very low coefficient of friction. One method for calculating the coefficient of friction between a vehicle tire and a road surface is disclosed in U.S. Pat. No. 4,779,447, issued to Rath. The Rath method uses a brake torque measurement to control vehicle brakes. However, since the method is not applicable to a traction control system, an additional calculation of the coefficient of friction is necessary for vehicle traction control.
Another example of a braking pressure control method is disclosed in U.S. Pat. No. 4,763,260 issued to Sakuma et al. In the Sakuma et al reference, ratios of various vehicle and engine conditions are used and an empirical estimation of the coefficient of friction is made in an antiskid control device. However, the ratios and the empirical friction coefficient estimation assume uniform, unchanging, flat road surface slip characteristics. Also, the values provided by the system are not made available for use with any other control devices, such as vehicle traction control.
In U.S. Pat. No. 4,794,538, issued to Cao et al, a method of controlling vehicle wheels to prevent skidding on a variety of surfaces is disclosed. The Cao et al reference measures wheel speed and braking pressure to calculate the coefficient of friction and, based on the calculated value of the coefficient of friction, adjusts wheel spin by controlling the braking pressure. However, a disadvantage to the method disclosed in Cao et al is that a friction-slip curve characteristic is assumed, with a fixed shape. With changing road conditions, the method may not necessarily be consistent with the laws of nature as operating conditions jump among a family of such curves, whereas the method assumed one such fixed curve. Additionally, the method does not consider whether or not the vehicle is on a sloping surface when controlling the braking pressure. Finally, the determined values are not made available for use with other control devices.
One antiskid device which also attempts to include hill slope in its brake control device is disclosed in U.S. Pat. No. 4,725,102, issued to Ando et al. In the Ando et al reference, the brake control device includes a hardware configured hill slope sensor. However, since the system does not output a coefficient of friction value or an actual hill slope value, the measurements and calculations are not available for use with other control devices.
Finally, U.S. Pat. No. 4,693,522, issued to Wupper et al includes a friction coefficient identification circuit to estimate the road surface coefficient of friction. However, the Wupper et al reference has no traction control application, as it is strictly an anti-skid position system. Additionally, the Wupper et al reference does not disclose a hill slope estimator or any other method of determining hill slope and including that determination in the braking control system.
It is seen then that there is a need for a method and apparatus capable of providing estimates of various external conditions from vehicle and engine measurements, which estimations can be made available for use in a variety of control applications.